Originally Posted by Chombi1

Those small plastic (and sometimes rubber) donuts are used to protect a bike's frame from getting scratched by bare cables slapping or rubbing against them. They are not really for reducing friction between the cable and the frame. They might protect the paint at the openings of the cable guides, but they do not really help that much to reduce friction or protect the paint from cable abrasion on the rest of the cable guides length. If you want to reduce friction between the cables and the bottom bracket cable guides, you should use a section of plastic tubing that you can pull out from brake cable casings that have them.